EP3776778A1

EP3776778A1 - Method and device for protection in a multi-terminal power transmission system

Info

Publication number: EP3776778A1
Application number: EP19721811.8A
Authority: EP
Inventors: Obbalareddi Demudu NAIDU; Neethu GEORGE; Swaroop GAJARE
Original assignee: ABB Power Grids Switzerland AG
Current assignee: Hitachi Energy Ltd
Priority date: 2018-03-31
Filing date: 2019-03-29
Publication date: 2021-02-17
Anticipated expiration: 2039-03-29
Also published as: CN111937264A; US20210143633A1; EP3776778B1; CN111937264B; US12021374B2; WO2019186490A1

Abstract

The present invention discloses a method and device for protection in a multi-terminal power transmission system. The device obtains measurements of pre-fault and during-fault currents for each terminal of the power transmission system and calculates pre-fault and during-fault values of positive sequence current phasors from the current measurements. At least two values of a fault section index are calculated for each section. Each fault section index value is calculated based on an assumed value for fault location, a difference between the pre-fault and the during-fault values of the positive sequence current phasors for two terminals, a length of the section, source impedances of the two terminals, and line parameters of the corresponding section. The faulted section is identified based on a comparison of the fault section index values calculated for each section, and a switching device is controlled according to the identity of the faulted section.

Description

METHOD AND DEVICE FOR PROTECTION IN A MULTI-TERMINAL POWER

TRANSMISSION SYSTEM

TECHNICAL FIELD

The present disclosure relates in general to fault localization in power transmission systems. More specifically, the present invention relates to fault localization in a multi-terminal transmission system using only current measurements.

BACKGROUND

Multi-terminal power transmission lines are used to integrate renewable power, or to supply power, to bulk industrial loads. Multi-terminal or tapped mixed lines provide a feasible solution to provide power at minimum cost. Multi-terminal mixed or tapped lines are transmission lines comprising of both overhead lines (OHL) and cables that are usually underground (UGC), that are connected at one or more taps or junctions. Using multi terminal/tapped mixed lines saves the costs associated with building substation and installing measurement transformers at tap/junction points. There has been a huge growth in the number of multi-terminal/tapped lines mainly due to increased renewable integration to the grid.

Auto-reclosing is vital for the reliability of power system. Introducing tap in a main overhead line (OHL) and connected underground cable (UGC) complicates the auto-reclosing philosophy. To define the auto-reclosing of a multi-terminal mixed line, fault section identification information is required. In general, when the fault is in an OHL, auto-reclosing is permitted for two or three times as most of the faults in OHL are transient faults. On the other hand, for fault in cables, auto-reclosing is not permitted because majority of the cable faults are rather of a permanent nature. The case becomes more challenging in case of multi terminal mixed lines where the auto reclosing is to be enabled only for a fault in an overhead line and blocked for a fault in a cable.

In order to accurately identify whether the fault is in an overhead line or in a cable, typically voltage and current measurements are required. This adds an overhead in case of multi-terminal lines as there are three or more terminals and measurement equipment are to be provided for each terminal and tap/junction points. These power systems may or may not have both voltage and current measurements at each terminal. Especially, there may not be measurement equipment at each tap / junction point. There is accordingly a need for determining the fault section with limited measurements at various locations of the multi-terminal power transmission system, in order to accurately enable auto-reclosing in such systems.

Accurate fault location in such transmission lines is equally important for maintenance crew to reach the fault point and undertake repair quickly. Quick identification of fault location improves the reliability, availability and save the revenue loss for the utilities.

In case of multi-terminal lines, numerous methods are employed to identify fault location. Known techniques for determining fault location typically rely on both voltage and current measurements, negative sequence quantities and knowledge of fault type or fault loop information. In multi-terminal mixed lines, such measurements or knowledge of fault type or fault loop may not be available. Also, methods based on specific fault type or fault loop information, or specific measurements may not work for all fault cases, or power system configurations.

Thus, in addition to determining the fault section with limited measurements, it is desired to have accurate fault location with limited measurements and fault information.

SUMMARY

Various aspects of the present invention relate to protection in response to a fault or fault protection in a multi-terminal power transmission system (power transmission system). In accordance with various embodiments, the power transmission system includes one or more overheard lines and one or more cables, wherein two or more transmission lines are connected at one or more tap or junctions. In other words, the power transmission system is a mixed or hybrid transmission system having a combination of overhead lines and cables. Further, the transmission system can have single or double circuit lines connecting different power systems.

The power transmission system connects three or more terminals, with at least a first terminal, a second terminal and a third terminal. Here the three or more terminals are connected by two or more transmission lines, with at least a first transmission line and a second transmission line. The two or more transmission lines are connected at one or more tap or junction points. For instance, the first and second transmission lines are connected at a junction. Thus, various line sections (sections) are formed between terminals and taps / junctions. In case of a three terminal system having three terminals connected by two transmission lines at a junction, there are three sections - a first section between the first terminal and the junction, a second section between the second terminal and the junction, and a third section between the third terminal and the junction. Also, at least one of the two transmission lines is an overhead line and the other a cable. Accordingly, one or more sections are overhead line sections or cables.

The present invention discloses a method for protection in response to a fault (or disturbance) in the multi-terminal power transmission system. The fault (or disturbance) can be in a section of the one or more sections of the power transmission system. Such a fault can happen due to a temporary disturbance (e.g. due to bad weather conditions), due to insulation failure and so forth. The method determines which section of the power transmission system has the fault, using only current measurements, to correctly perform auto-reclosing operations. The method can also determine the location of the fault (i.e. distance from a terminal) on the section.

The method is performed with an Intelligent Electronic Device (IED) associated with the power transmission system. For example, the method can be implemented with an IED associated with a terminal such as one of the first, second and third terminals of the system.

The method includes obtaining pre-fault and during-fault measurements of currents for each terminal of the power transmission system. The current measurements can be obtained using measurement equipment such as current transformers, sensor-based measurement equipment (e.g. Rogowski coils, non-conventional instrument transformers etc.) and the like, which provide a signal corresponding to currents as sensed from the line. The IED receives the signal from the measurement equipment and processes the same to obtain the pre-fault and during-fault current measurements. Alternately, another power system device such as a disturbance recorder can process the current signals and provide the same to the IED.

The method further includes calculating pre-fault and during-fault values of positive sequence current phasors from the pre-fault and during-fault measurements of currents for each terminal. The IED can calculate current phasors (e.g. using suitable phasor calculation such as Fourier calculations etc.), from the current measurements. Positive sequence quantities can be derived by using methods such as symmetrical component analysis etc. In addition, the method includes calculating at least two values of a fault section index for each section. For example, two values for a fault section index can be calculated for one section. For calculating the value for a section, the following are utilized:

• an assumed value for fault location;

• a difference between the pre-fault and the during-fault values of the positive sequence current phasors for two terminals;

• a length of the section;

• source impedances of the two terminals; and

• line parameters of the corresponding section.

The assumed value for the fault location can be a value of‘O’, in case the fault location is assumed to be on the beginning of the section,‘ 1’ in case the fault location is assumed to be at the end of the section. Thus one value can be calculated for‘O’ and another value can be calculated for‘ G, where 1 corresponds the length of the section.

In some embodiments, several values for fault location are assumed. This can be starting from‘O’ and incremented in small amounts (e.g. 0.1). Here, identifying the section having the fault includes calculating a location of the fault by calculating a plurality of values for the fault section index based on the plurality of assumed values for the fault location.

The difference between the pre-fault and during-fault positive sequence current phasors is calculated for the phasors of two terminals. The terminals include the terminal of the section, and one other terminal. Consider a system having terminals A, B and C, and sections AJ, BJ and CJ, where AJ is the section between terminal A and junction J, BJ is the section between terminal B and J, and CJ is the section between terminal C and J. Here, for section AJ, the phasors for terminal A and that of terminal B or C are used.

The assumed value, the difference between the current phasors, are used with source impedances of the two terminals and line parameters of the corresponding section. For example, for section AJ, fault section index (K_A0, K_A1) can be calculated as follows:

Here, K_A0 is with assumed value of‘O’ for the fault location, and is for an assumed value of‘ G for the fault location. The difference between pre-fault and during-fault positive sequence current phasors of terminal B is I_B— I_B' ^pred^, nd terminal A is I_A— l_A ^pre Further, Pl, Ql, S l and Tl are calculated using the source impedances and line parameters as follows:

The ABCD parameters and source impedances can be used for calculating Pl, Ql, S l and

Tl:

tanh ( _BJL_BJ * 0.5) +

^AB> ^~ TC B] Z^cBJ*sinh(Y_BjL_Bj)

Z^cBJ*sinh(y_BJL_Bj)

In the above,

• Z mn^d is the corresponding element of the modified bus impedance matrix after simply removing the elements corresponding to the faulted section AJ

• Z _j is the corresponding element of the original bus impedance matrix

• Zc_{B j} is the surge impedance of line B J, g_{B j} and L_{B j} are respectively

propagation constant and lengths if the line BJ • Zc_A i is the surge impedance of line AJ, g_A] and L_AJ are respectively

propagation constant and lengths if the line AJ

The values calculated for the fault section index for each section are used for identifying the section having the fault. The section having the fault is identified based on a comparison of the values of the fault section index estimated for each section. For example, values of K_A0, and K_A1 are compared for section AJ, similarly values computed for section BJ are compared, and so forth. A criteria can be used for identifying the section with the fault. For example, when the two values (in case two values are computed) are of different signs (opposite polarity), then the corresponding section is determined to have the fault.

In the embodiments where several values for the fault section index are calculated, the plurality of values for the fault section index are compared with a threshold. The value which is closest to the threshold (e.g. less than or equal to the threshold) is considered to determine the fault location. The fault location corresponds the assumed value of the fault location, where the value for the fault section index is less than the threshold. Thus, this assumed value of fault location is selected as the fault location.

The identity of the fault section (i.e. it being an overhead line section or a cable) is used for controlling a switching device according to the identification. Here, auto-reclose is enabled based on the section being an overhead line section.

Thus, the IED at a terminal can receive the various current measurements and perform the various steps of the method to determine the faulted section (section having the fault). The IED has an interface for receiving the signals from the measurement equipment, and measurements from other IEDs (e.g. through wired / wireless communication). The IED has a processor configured to perform various steps involved in determining the faulted section. The processor can also performs steps in relation to determining the location of the fault on the section. The processor performs these steps based on information received on the interface and that stored on a memory of the IED. For instance the source impedances and line parameters can be stored in the memory and utilized with the pre-fault and during-fault information, to arrive at the faulted section or fault location. In an embodiment, the method also includes calculating the source impedances for each terminal. Here, the source impedance for each terminal is calculated based on status of one or more circuit breakers connected at the terminal. The status of the circuit breakers is determined from a communication signal (e.g. a GOOSE communication). An admittance value of a circuit element connected through each circuit breaker is considered based on the status of the circuit breaker. For example, when the breaker is connected, the admittance value of the circuit element is considered. Otherwise, it is not considered. In accordance with the embodiment, the processor also calculates the values of the source impedances for each terminal. These are based on the status signals received at the interfaces.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 shows a three terminal tapped power transmission system, in accordance with an embodiment of the invention;

Figure 2 is a simplified block diagram of an intelligent electronic device, in accordance with various embodiments of the invention;

Figure 3 shows the three terminal system of Fig. 1 along with rest of the transmission network connected at the three buses;

Figure 4 is an equivalent pi model of a section BJ of the three terminal system;

Figure 5 is an equivalent pi model of a section AJ of the three terminal system with fault at F, dL kilometres from bus A;

Figure 6 is a plot of a fault section index (KA) for various fault locations; and

Figure 7 is a flowchart of a method for protection in response to a fault, in accordance with various embodiments of the present invention. DETAILED DESCRIPTION

Various aspects of the present invention relate to protection in response to a fault or fault protection in a multi-terminal power transmission system (power transmission system). This is a mixed / tapped power transmission system.

An example of such a mixed / tapped power transmission is shown in Fig. 1. The transmission system shown in Fig. 1 is a three-terminal/tapped power transmission system. Bus A, Bus B and Bus C are the three terminals and J is the junction. Section AJ is an overhead line section of length LOHLI km (or LAJ), section BJ is an overhead line section of length LOHL2 km (or LBJ) and section CJ is a cable (underground) of length LUGC km (or LCJ). The power transmission system shown in Fig. 1 will be relied on for explaining the method and device of the present invention. The invention however is not limited to the three-terminal system as shown in Fig. 1 and can be extended to other systems such as power systems having four or more terminals and single / double circuit lines, by making appropriate adaptations to the method / device for such systems.

The present invention discloses a method for protection in response to a fault (or disturbance) in case of such power transmission systems. The fault (or disturbance) can be in a section of the one or more sections of the power transmission system. For example, the fault can be on AJ, BJ or CJ. Such a fault can happen due to a temporary disturbance (e.g. due to bad weather conditions), due to insulation failure and so forth.

The method determines which section (AJ, BJ or CJ) of the power transmission system has the fault, using only current measurements, to correctly perform auto-reclosing operations. The method can also determine the location of the fault (i.e. distance from a terminal) on the section. In order to determine the faulted section and the fault location, synchronized current measurements for all terminals before the fault (pre-fault) and during the fault (as and when the fault is detected) are required.

The method is performed with an Intelligent Electronic Device (IED) associated with the power transmission system. For example, the method can be implemented with IED1 associated with terminal A or IED2 associated with terminal B. Each IED as shown in Fig. 1 has measurements for all terminals. For example, IED1 receives the current signals from current transformer (CT1) shown in Fig. 1 and obtains the current measurements for Bus A from the signals. Further, IED1 is in communication with other IEDS (IED2 and IED3) through a communication channel such as a fiber optic channel shown in Fig. 1. Thus, IED1 also has measurements for other terminals via communication from IED2 and IED3. The three (or more) IEDs of the power transmission system are synchronized with each other. This can be by synchronizing the clocks of these IEDs. Thus, the current measurements at all terminals are synchronized with each other.

An IED such as IED1 has an interface (202), a memory (204) and a processor (206), among other components, as shown in the simplified representation of an IED in Fig. 2.

The interface (202) receives signals from the measurement equipment, for example from CT1 in case of IED1. There could be separate I/O for receiving the current signals and communication signals from other IEDs. Each IED in accordance with the invention has one or more interfaces to receive / transmit signals. The memory (204) stores various information necessary to perform the method of the present invention. These can include measurements, source impedances, line parameters including line lengths, impedances etc. These can also include instructions that executable by the processor (206) to perform the steps of the method disclosed herein. The information in the memory can be uploaded to the IED, or calculated and stored during runtime. The processor (206) performs various steps of the method based on the information received via the interface using the information present in the memory.

The following describes the method of the invention, various steps of which are implemented using the IED (such as IED1, IED2 etc.) described hereinabove. The method will be described considering the example power transmission system shown in Fig. 1. Also, ABCDA_J shall denote the positive sequence line impedance parameters (ABCD matrix) of section AJ. ABCD_BJ denotes positive sequence line impedance parameters of section BJ. ABCD_CJ denotes positive sequence line impedance parameters of section CJ. Let us consider the fault at section AJ. In order to perform the method, source impedances for the terminals are required. The actual source impedances can be used. Alternately, the source impedances can be calculated from breaker status and line impedance data.

Considering the representation of the three terminal system as shown in Fig. 3, the bus admittance matrix of the equivalent network can be written as below:

The diagonal elements y_aa, y_jj, y_bb and y_cc are calculated as the sum of all the admittances connected to the respective buses. For example, y_aa is the sum of all the admittances connected to bus A. This shall depend on the status of the circuit breakers at the bus which can be received using a communication such as a GOOSE/GSSE or wired communication, namely SA1, SA2 ... to SAn. If the breakers are closed, the corresponding impedances are connected to the bus A and thus has to be included in the calculation of element yaa. The series line impedance and shunt admittance of section AJ which are also connected to bus A through closed breaker SA are also included in the calculation of y_aa.

In the calculations, the source impedance behind any bus is considered to be the equivalent of the impedances connected to the bus. This can be the impedances of the lines connected to the bus or the impedance of the transformer. In case of transformers, the transformer impedance is sufficient; and the LV voltage equivalent is not required. For example, if all the breakers at bus A are closed, the element y_aa can be calculated as,

1

If any of the breakers, for example, breaker SA3 is opened, then the term— shall be

^ZA3 removed from equation (2). Also, if any additional compensation devices are added to the system, the details shall be acquired and the corresponding impedance term can be added to the admittance calculation in equation (2). For example, if a series compensation device is added behind SA1 with may be 50% compensation, then correspondingly 0.5 times the effective impedance of the device can be incorporated to equation (2).

Similarly, yjj, ybb and ycc can be calculated as below in equations (3), (4) and (5)

Thus, the method utilizes the source impedances - either that are uploaded beforehand to the IED, or by calculating the source impedances as described above.

The method involves calculating pre-fault and during-fault positive sequence current phasors at all terminals by suitable phasor estimation method. Current measurements are available from the IEDs, disturbance recorders or other such devices, at all the three terminals. Pre-fault and post fault positive sequence current phasors are calculated from these using any suitable phasor estimation method.

For the three terminal system, pre-fault and post fault positive sequence currents at terminal A are denoted by lA^{1 ,pre} and IA¹ respectively, pre-fault and post fault positive sequence currents at terminal B is denoted by lB^{l pre} and IB ¹ respectively, and pre-fault and post fault positive sequence currents at terminal C is denoted by Ic^1,pre and Ic¹ respectively.

In order to determine the fault section and fault location, values for fault section index K are calculated using source impedances, line parameters, and the current phasors. The following describes how the fault section index is arrived at.

Initially, fault component of positive sequence current at any bus corresponding to any one healthy section is calculated. Assuming fault on section AJ, this step involves calculation of (/_g as a function of impedances and unknown fault section identification (d).

Considering the positive-sequence pure-fault network, the post-fault voltage at bus B of the system can be expressed as

E = V_{g '} ^pre - Z ^w(d) * l_F (6) where, V_B and V_{B '} ^pre are post-fault and pre-fault voltages at terminal B respectively,

I_F is the fault current through the fault resistance, and

Z_Bp ^W is the BF element of the new bus-impedance matrix which includes the fault bus F, and is calculated as below:

¾T(d) =

_ (ZIB/ ^sinh(YAj*LAj*d)) + (z°_B/ ^sinh(Y_AJ*L_Aj*(l-d))) _

(l/ sm\\(y_Aj*L_Aj*d )+(l/ sinh(y^ *L^ *(l-d)))+tanh(y^ *L^ *ci*0.5)+tanh(y^ *L^ *(l-ci)*0.5) In equation (7),

Z^_B and Z_jB are the corresponding elements of the original bus impedance matrix (either available in the IED or calculated as described above), y_Aj and L_AJ are propagation constant and length of faulted section respectively, and

Ip is the fault current.

Equation (6), along with (7), expresses post-fault bus voltage V_B as a function of fault location d.

Considering the distributed parameter model of healthy section BJ, as shown in Fig. 4, the positive-sequence component of the post-fault current at bus B can be obtained by Kirchhoff s voltage law (KVL) and Kirchhoff s current law (KCL) as

Vi

i = tanh (y_BJL_BJ * 0.5) +

Zc BJ Zc_BJ * sinh (Y_BJL_bj) where,

Zc_{B j} is the surge impedance of line B J, g_{B j} and L_{B j} are respectively propagation constant and lengths of the line section BJ, and V_B and V ¹ are post-fault positive sequence voltages at buses B and J.

Referring to equation (6) and substituting in equation (8), and rewriting we get where I^'^pre is the pre-fault positive sequence current at bus B of line BJ, and A_BJ and B_BJ can be calculated as below,

(10) (11)

BJ— Zc_BJ * sinh (Y_B]L_bj)

The pure-fault current, i.e. difference between post- and pre-fault current phasors at bus B corresponding to one of the two healthy sections can therefore be expressed as in (12) as a function of unknown fault distance d, impedances and fault current I_F

Similarly, pure-fault current, i.e. difference between post- and pre-fault current phasors at bus C can also be written as in (13)

Thereafter, fault component of the positive sequence current at bus corresponding to faulted section is calculated.

Fig. 5 shows section AJ between bus A and junction J with a fault at point F. The fault is at a distance d from the bus A, where d is expressed as a fraction of the total length of the faulted line, LAJ kilometres. Here, the distributed parameter model of the line is depicted by an equivalent pi model. Assuming fault on section AJ, this step involves calculation of (I — I_A ^pre) as a function of impedances and unknown fault location (d).

A_AJ and B_AJ correspondingly for the faulted line cannot be developed by (10) and (11), since the line impedance can no more be considered as a constant. To obtain A_AJ and B_AJ , we remove line AJ from the system and replace it by two current sources. Considering the positive sequence superimposed network for this modified system, the following equations can be formed, where,

Z _j ^0d, Zj ^od , Z ^d and Zjj^od is corresponding elements of the bus-impedance matrix of the reduced network excluding faulted section AJ. This can be simply formed by deleting the terms corresponding to this section in the YBUS formulation, or from the source impedance matrix with the IED,

V¹, V_{A '} ^pre are post-fault and pre-fault voltages at bus A,

V¹, V_j ^1,pre are post-fault and pre-fault voltages at J,

I_Aj&nd I_A†^re are post-fault and pre-fault currents at bus A, and

¾ and lj_A ^pre are post-fault and pre-fault currents at J flowing into section AJ.

For faulted section AJ,

Equating equations (14) to (16) and (15) to (17),

Now solving from (18) and (19), we get,

Comparing (20) and (21) with (13), we get

_ _'/mod

A] (23)

B_AJ BID

ymod ² _ ymod ymod

DL] ^d]] ^dAA

From equations (20) and (21), the following equation (24) can be written for the faulted section AJ, similar to equation (12)/ (13)

The quantities estimated above are equated with measured quantities to solve for the fault section identification. Assuming that the fault is on section AJ, rewritten below, for convenience, are equations (12), (13) and (24) derived in previous sections

Here, LHS of the above three equations shall be calculated from the current measurements at buses A, B and C.

Taking ratios of equations (25) and (27), we get l_B - I¹ " ₌ [ _¾Z¾T(<0 + B„,Z?r(d)] (28)

/ⁱ - I¹ " [A_A,Z^(d) + ByZ TW] where,

Z_gp ^W(d), Z _p (d), Z^_p (d) and Z†_p (d) can be calculated from equation (7), A_BJ and B_BJ can be calculated from equations (10) and (11), and A_AJ and B_AJ can be calculated from equations (22) and (23).

Substituting for terms in equation (28), we get,

P * sinhfay * L_AJ * d) + Q * [sinhfay * L_AJ) coshfay * L_A] * d ) - coshfay * L_AJ) sinhfay * L_A] * cl)] T * [sinh^y * L_AJ ) cosh(]^y * L_A] * d ) - cosh(]^y * L_AJ) sinh^y * l_AJ * d)] + S * sinh^y * L_AJ * d )

(29)

where,

P— A_BJZJ_B + B_BJZJJ ; Q— A_BjZ_BB + B_BJZ_BJ

S— A_AJZJ_A + B_AJZJJ T— A_AJZ_BA + B_AJZ_BJ

This can be used to define fault section index for each section (KA for AJ, K_B for BJ, Kc for CJ. For a fault on section AJ, referring to equation (29), we know that

P * sinhfay * L_AJ * d) + Q * [sinhfay * L_A] ) coshfay * L_A] * d ) - coshfay * l_AJ) sinhfay * L_A] * d)]

T * [sinh^y * L_aj) cosh^y * L_A] * d ) - cosh^y * l_AJ) sinh^y * L_A] * d)] + S * sinh^y * L_A] * d )

(30)

where,

PI = A_BJZ + B_BJZf_j ; Q1 = A_B]Z_B°_B + B_BJZ_b°_j·

SI = A_AJZJ_A + B_AJZf_j ; T1 = A_AJZ_B°_A + B_A]Z_B°_J

-1

A_BJ — tanh(y_SyL_Sy * 0.5) +— ; B_BJ —

^ZcBJ ^ZcBJ*smh(Y_BjL_Bj) Zc ^‘ B J*sm (Y_B j L_B j)

In the above,

Z™^d is the corresponding element of the modified bus impedance matrix after simply removing the elements corresponding to the faulted section AJ,

Z _j is the corresponding element of the original bus impedance matrix,

Zc_{B j} is the surge impedance of line BJ, g_{B j} and L_{B j} are respectively propagation constant and lengths if the line BJ, and Zc_Aj is the surge impedance of line AJ, g_A} and L_AJ are respectively propagation constant and lengths if the line AJ.

Fault section index K can be defined in one way as below in equation (30)

K_A

T_jl _ .pre- _ ‘B ‘B

~ _jl _ .pre

L -I

_ PI * sinhfa_y * L_AJ * d) + Q1 * [sinhfay * L_AJ) coshfay * L_A] * d ) - coshfay * L_AJ) sinhfay * L_A] * cl)]

T1 * [sinh^y * L_aj ) cosh^y * L_A] * d ) - cosh^y * l_AJ) sinh^y * L_A] * d )] + 51 * sinh(^y * L_A] * d )

It is observed that KA as calculated using equation (30) is equal to zero when the variable, d, is equal to the unknown fault location expressed as a fraction of length of section AJ. For values of d less than the actual fault location, the value of KA will be positive. For values of d greater than the actual fault location KA will be negative, as seen in Fig. 6.

Hence by checking the value of K_A at d=0 and d=l and comparing their signs, we can identify if there is a fault on section AJ. Let us create a fault in 50% of AJ section. As seen from Fig. 6, if the fault is on section AJ, the value of K_A0 will be positive and value of K_A1 will be negative.

K_A0 is defined as K_A at d=0 and K_A1 is defined as K_A at d=l. From equation (30),

Similarly, for a fault on section BJ, we get equation (31)

P 2 * sinh(y_Sy * L_BJ * d) + Q2 * [sinh(y_Sy * L_B ) cosh(y_Sy * L_BJ * d)— cosh(y_Sy * L_B ) sinh(y_Sy * L_BJ * d) T2 * [sinh(y_Sy * L_BJ ) cosh(y_s; * L_BJ * d) - cosh (y_s; * L_gj ) sinh (y_s; * L_BJ * d)] + 52 * sinh(y_s; * L_gj * c where,

P2 = A_BJZJ_B + B_B]ZJJ ; Q2 = A_BJZ_B°_B + B_BJZ_b°_j·

S2 = A_AJZ _A + B_A]ZJJ ; T2 = A_AJZ_B°_A + B_A]Z_b°_j·

Z_jB> Z_jj, Z_BB , Z_Bj, Z_jA and Z_BA are the corresponding elements of the original bus impedance matrix

In the above,

Z™°^d, Z] ^od , Z™_B ^d and Zjj^od is corresponding elements of the bus-impedance matrix of the reduced network excluding faulted section BJ. This can be simply formed by deleting the terms corresponding to this section in the YBUS formulation.

ZC_BJ is the surge impedance of line BJ, y_Bj and L_BJ are respectively propagation constant and lengths if the line BJ, and

Zc_Aj is the surge impedance of line AJ, g_A} and L_AJ are respectively propagation constant and lengths if the line AJ.

By checking the value of K_B at d=0 and d=l and comparing their signs, we can identify if there is a fault on section BJ. If the fault is on section BJ, the value of K_B0 will be positive and value of K_B1 will be negative.

K_B0 is defined as K_B at d=0 and K_B1 is defined as K_B at d=l.

From equation (

Similarly, for a fault on section CJ, we have equation (32),

_ P 3 * Sinh(y_c; * L_Cj * d) + Q3 * [sinh(y_c; * L_C] ) cosh(y_c; * L_C] * d) - cosh ( _CJ * L_C] ) sinh(y_c; * L_C] * d)] T 3 * [sinh(y_C * L_C] ) cosh(y_cy * L_C] * d) - cosh (y_CJ * L_C] ) sinh(y_cy * L_C] * d)] + 53 * sinh(y_cy * L_C] * d) where,

P3 = A_CjZf_c + B_C]Zf_j ; Q3 = A_C]Z_C°_C + B_C]Z_C°_]· S3 = A_AJZ _A + B_A]ZJJ ; T3 = A_AJZ_C°_A + B_AJZ_c° Z_jC, Z _j, Zee _> Z , Z _A and Z_BA are the corresponding elements of the original bus impedance matrix

Z™^d, Z™^d , Z™°^d and Zjj^od is corresponding elements of the bus-impedance matrix of the reduced network excluding faulted section CJ. This can be simply formed by deleting the terms corresponding to this section in the YBUS formulation.

1 , . 1 -1

tanh (Y_A]L_AJ * 0.5) +

^Aa‘ ^~ Zc B_AJ—

A] Zc_A] * svah{y_A]L_A]y Zc_A] * sinh (y_AJL_Aj

Zc_Cj is the surge impedance of line CJ, y_Cj and L_CJ are respectively propagation constant and lengths if the line CJ, and

K_Co is defined as Kc at d=0 and K_C1 is defined as Kc at d=l.

From equation (

By checking the value of Kc at d=0 and d=l and comparing their signs, we can identify if there is a fault on section CJ. If the fault is on section CJ, the value of K_CQ will be positive and value of K_C1 will be negative

Thus, for the faulted section can be identified as follows

• Calculate constants Pl, Ql, Sl and Tl as given in previous step

Calculate

Calculate constants P2, Q2, S2 and T2 as given in previous step

Calculate

Calculate constants P3, Q3, S3 and T3 as given in previous step • Calculate

Compare the value of K for each section, and identify the faulted section as follows:

• If K_AO and K_AI are of the opposite sign, the fault is on section AJ

• If KBO and KBI are of the opposite sign, the fault is on section BJ

• If Kco and Kci are of the opposite sign, the fault is on section CJ

Thus, by calculating the fault section index K as shown, the faulted section can be identified. The fault section index may be calculated differently by having different assumptions and different assumed values for d.

The fault location can be identified by having different values calculated for K. For example, by varying the value of d, different values of K can be calculated for a section. The value of‘d’ for which K is close to a threshold (in the above case‘0’) is determined the fault location.

The identified section comprising the fault can also be considered as an input to calculate the location of fault in the identified section.

The equation (28) can also be written as follows:

where

cosh(y_Af * L_AJ)— Pl + Ql CQS]I(;/₄ * l_A})

)— Rl * G1 smh( y_AJ * L_Af)

The value of (d) can also be obtained by considering any healthy section and a faulted section in the multi-terminal transmission line system. Thus, the location of the fault in the faulted section can be identified using only current measurements, line parameters and source impedances.

The method for protection based on the fault section identification is shown in flow chart of Fig. 7.

As shown, at 702, the method includes obtaining pre-fault and during-fault measurements of currents for each terminal of the power transmission system.

The method further includes at 704, calculating pre-fault and during-fault values of positive sequence current phasors from the pre-fault and during-fault measurements of currents for each terminal. The IED can calculate current phasors (e.g. using suitable phasor calculation such as Fourier calculations etc.), from the current measurements. Positive sequence quantities can be derived by using methods such as symmetrical component analysis etc.

At 706, the method includes calculating at least two values of a fault section index for each section. For example, two values for a fault section index can be calculated for one section. For example, for section AJ, fault section index (K_A0, K_A1) can be calculated as explained above, and similarly for BJ and CJ.

The values calculated for the fault section index for each section are used for identifying the section having the fault. The section having the fault is identified based on a comparison of the values of the fault section index estimated for each section at 708. For example, values of K_A o, and K_A1 are compared for section AJ, similarly values computed for section BJ are compared, and so forth. A criteria can be used for identifying the section with the fault. For example, when the two values (in case two values are computed) are of different signs (opposite polarity), then the corresponding section is determined to have the fault.

Several values for the fault section index can be calculated for determining a fault location. The plurality of values for the fault section index are compared with a threshold. The value which is closest to the threshold (e.g. less than or equal to the threshold) is considered to determine the fault location. The fault location corresponds the assumed value of the fault location, where the value for the fault section index is less than the threshold. Thus, this assumed value of fault location is selected as the fault location. This can happen subsequent to auto-reclosing.

The identity of the fault section (i.e. it being an overhead line section or a cable) is used for controlling a switching device according to the identification at 710. Here, auto-reclose is enabled based on the section being an overhead line section. Thus, a circuit breaker associated with an overhead line section can close again if it is decided that the fault is in the overhead line. The IED (processor) can generate a signal for auto -reclosing with the circuit breaker accordingly.

Thus, the IED at a terminal can receive the various current measurements and perform the various steps of the method to determine the faulted section (section having the fault). The IED further controls a switching device such as a circuit breaker to enable auto-reclose only for overhead line section faults. Additionally the fault location is calculated using only current measurements.

Thus, the present invention provides a method and device for estimating fault section and location in a multi-terminal transmission line system using only current measurements. Also, line parameters and source impedances are required, where source impedances can be calculated if not available.

The proposed method is non-iterative and the accuracy of the method is not affected by fault resistance, fault loop information, loading conditions, mutual coupling and line configuration.

Claims

1. A method of protection in response to a fault in a multi-terminal power transmission system, wherein the multi-terminal power transmission connects at least a first terminal, a second terminal and a third terminal, with at least a first transmission line connecting the first terminal and the second terminal, and a second transmission line connecting the third terminal and the first transmission line at a junction of the first and second transmission line, forming a section between each terminal and the junction on the first and second transmission lines, wherein each section is one of an overhead line section and a cable, and the fault is located in one of the sections, wherein the method is performed with an Intelligent Electronic Device (IED) associated with one of the first, second and third terminals of the multi-terminal power transmission system, the method comprising:

obtaining pre-fault and during-fault measurements of currents for each terminal of the multi-terminal power transmission system;

calculating pre-fault and during-fault values of positive sequence current phasors from the pre-fault and during-fault measurements of currents for each terminal;

calculating at least two values of a fault section index for each section, wherein each value of the fault section index for a section is calculated based on:

an assumed value for fault location;

a difference between the pre-fault and the during-fault values of the positive sequence current phasors for two terminals of the power transmission system, wherein the two terminals include the terminal that the section is connected at and one of the other terminals of the multi-terminal power transmission system;

a length of the section;

source impedances of the two terminals; and

line parameters of the corresponding section;

identifying the section having the fault based on a comparison of the values of the fault section index estimated for each section; and

controlling a switching device according to the identification of the section having the fault, wherein auto-reclose is enabled based on the section being an overhead line section.

2. The method as claimed in claim 1, wherein identifying the section having the fault comprises calculating a location of the fault by:

calculating a plurality of values for the fault section index based on a plurality of assumed values for the fault location;

comparing the plurality of values for the fault section index with a threshold; and selecting the assumed value of the fault location as the location of the fault, where the value for the fault section index is less than the threshold.

3. The method as claimed in claim 1, wherein the source impedance for each terminal is calculated based on status of one or more circuit breakers connected at the terminal, wherein the status of the circuit breakers is determined from a communication signal, and an admittance value of a circuit element connected through each circuit breaker is considered based on the status of the circuit breaker.

4. An Intelligent Electronic Device (IED) configured for protection in response to a fault in a multi-terminal power transmission system, wherein the multi-terminal power transmission connects at least a first terminal, a second terminal and a third terminal, with at least a first transmission line connecting the first terminal and the second terminal, and a second transmission line connecting the third terminal and the first transmission line at a junction of the first and second transmission line, forming a section between each terminal and the junction on the first and second transmission lines, wherein each section is one of an overhead line section and a cable, and the fault is located in one of the sections, the IED comprising:

an interface for obtaining pre-fault and during-fault measurements of currents for each terminal of the multi-terminal power transmission system;

a memory; and

a processor configured to:

calculate pre-fault and during-fault values of positive sequence current phasors from the pre-fault and during-fault measurements of currents for each terminal;

calculate at least two values of a fault section index for each section, wherein each value of the fault section index for a section is calculated based on:

an assumed value for fault location; a difference between the pre-fault and the during-fault values of the positive sequence current phasors for two terminals of the power transmission system, wherein the two terminals include the terminal that the section is connected at and one of the other terminals of the multi-terminal power transmission system;

a length of the section;

source impedances of the two terminals; and

line parameters of the corresponding section;

identify the section having the fault based on a comparison of the values of the fault section index estimated for each section; and

generate a signal to control a switching device according to the identification of the section having the fault, wherein auto-reclose is enabled based on the section being an overhead line section.

5. The IED as claimed in claim 4, wherein the process is configured to identify the section having the fault by calculating a location of the fault, wherein the process is configured to calculate the location of the fault by:

6. The IED as claimed in claim 4, wherein the processor is configured to calculate the source impedance for each terminal based on status of one or more circuit breakers connected at the terminal, wherein the status of the circuit breakers is determined from a communication signal, and an admittance value of a circuit element connected through each circuit breaker is considered based on the status of the circuit breaker.